Chemically impregnated in situ regeneration of the efficiency of activated carbon filters for trapping radioactive iodine

ABSTRACT

A compound for chemically impregnating an activated charcoal bed for efficiently removing radioactive iodine from air flow from an area through a chemically impregnated carbon filter in a nuclear air cleaning system in which the compound is selected from Quinuclidine.CH3I; Triethylenediamine.2CH3I; and Hexamethylenetetramine.CH3I.

RELATED APPLICATION

This application is a continuation in-part of application Ser. No.506,790 filed on June 22, 1983, now U.S. Pat. No. 4,518,562.

This invention is directed to chemically impregnated nuclear-gradeactivated carbons used for trapping radioactive iodine and moreparticularly to a method of regenerating chemically impregnatednuclear-grade activated carbon in place.

The technology of activated carbons in air-cleaning application to thenuclear industry differs in two important respects from other activatedcarbon applications. First, a nuclear-grade activated carbon filter isused continuously for an extended period of time and must reduce theiodine-131 concentration to a safe level at any point in time, includingan accident mode. Second, the exhausted carbon is not regenerated as innon-nuclear applications, and this seriously influences the economy ofthe nuclear application.

The activated carbons for use in nuclear power plants are impregnatedwith one of several formulations in order to increase the efficiency ofthe adsorbent for trapping radioactive methyliodide-131. There arepresently three types of impregnant materials used in commerce for theabove purpose: potassium iodide (KI), selected tertiary amines such astriethylenediamine (TEDA), and the co-impregnation of KI and a tertiaryamine. The useful life of activated carbon filters in the ventilationsystems of nuclear power stations is slowly impaired by the contaminantscontained in the large volume of process air and the above commercialimpregnated carbons now in use have a limited life. Moreover, theycannot be regenerated. During service in nuclear power installations,the carbon filters are contacted with the ubiquitous air pollutants suchas sulphur dioxide, nitrogen oxides, and ozone; other species known tobe present are the volatile hydrocarbons and water vapor. As a result ofthe exposure, the carbon is progressively degraded in its efficiency toremove the test gas, radioactive methyliodide-131, and eventually theactivated carbon must be replaced.

Sizable quantities of contaminants enter a carbon filter. For example,an air flow of 30,000 cfm for 90 days through a filter containingapproximately 3,750 pounds of activated carbon (2-inch depth) canintroduce the contaminants listed in Table 1. Accordingly, for a designlife of 30 months, considerable quantities of contaminant can accumulatein the carbon filter.

                  TABLE 1                                                         ______________________________________                                        Some Contaminants Passing Into a 30,000 cfm Filter                                    Concentration                                                                            Weight                                                             ppm (V/V)  in 90 Days Wt. % of                                                3-Year AV. (lbs.)     Charcoal                                        ______________________________________                                        Ozone     0.019         10        0.27                                        SO.sub.2  0.023         16        0.43                                        Hydrocarbons                                                                            0.23         214 (Hexane)                                                                             5                                           (non-CH.sub.4)                                                                ______________________________________                                    

It has been determined that the commercial nuclear carbons are degradedby the atmospheric contaminants and the behavior was similar at threedifferent exposure sites which has been set forth in "Effects ofWeathering on Impregnated Charcoal Performance," NUREG/CR-2112 and NRLMEMO Report 4516, September 1981. The penetration of methyliodide-131 isgenerally considered as a measure of carbon-filter performance. Thepenetration rises rapidly in air flows greater than 70% relativehumidity, and the degradation correlates best with the dew point of theair flow.

The common air contaminants (ozone, sulfur dioxide, nitrogen oxides, andhydrocarbons) adversely influence the inlet layer of carbon in a filterbed, and these contaminants then migrate to lower depths with increaseof length of service. Vapors from solvent spills and organic vapors fromplant operations contribute to the degradation more strongly at higherhumidities than at lower humidities.

Pertinent to this invention, it has been determined that weatheredcarbons cannot be regenerated at ambient temperature by a continued airpurge.

It has been determined that chemically impregnated charcoals, weatheredto a degree wherein they are no longer useful, could be completelyregenerated by exposure to suitable radiation facilities.

Second, it has been determined that the adsorption of nonradioactivemethyliodide-127 was improved to a very small extent. This facteliminated the possibility that the irradiation had regenerated gasadsorption sites on the charcoal. However, since the trapping of theradioactive species, methyliodide-131, had been realized afterirradiation, it has been established that the improvement is due to theactivation of the isotope exchange capacity of the chemicallyimpregnated carbon.

Third, it has been determined that certain chemical compounds when addedto the gas flow entering the carbon filter will regenerate the exchangecapacity of exhausted chemically impregnated carbons. These observationsestablish the feasibility of restoring in situ the iodine-131 trappingof a chemically impregnated activated carbon bed. The improvement byradiation is thus attributable to chemical changes brought about duringthe radiolysis of the impurities on chemically impregnated servicecarbons.

This invention relates to an indefinite extension of the productive lifeof an activated carbon filter by improved chemically impregnating thecarbon and by introducinng controlled quantities of chemical reducingagents into the air flow entering the carbon filter. There are threedirect advantages to be gained: 1) saving in the new impregnated carbonrequired to replace exhausted material, 2) the savings in skilled laborrequired to replace the carbon and in the cost of waste burial, and 3)the assurance that a carbon filter is ready at any point in time to meetthe requirements of Regulatory Guide 1.52 of the U.S. Nuclear RegulatoryCommission.

In making this invention, it was observed that a number of improvedchemically impregnated carbons could be reactivated by exposure tohigh-level radiation to trap radioactive methyliodide-131. Oneexperiment using radiation exposures on the LINAC (Linear Accelerator)on commercial types of chemically impregnated carbon is summarized inTable 2.

                  TABLE 2                                                         ______________________________________                                        Penetration of Methyliodide-131 Before and After                              Linac Exposure of 10.sup.9 RADS (10-11 AUGUST 1981)                                            % Penetration                                                Sample   Weight (g)    Initial Radiated                                       ______________________________________                                        1        160           30.1    0.039                                          2        147           10.5    0.033                                          3        143           9.99    0.03                                           4        141           38.9    0.002                                          ______________________________________                                    

Sample 1 (Table 2) had been used for two years in glove-box operationswith iodine-125 for the preparation of organic compounds used in medicaltreatments. Sample 2 had been similarly used for one year. Sample 3 hadbeen weathered in a flow of outdoor air (1.05×10⁸ cubic feet) for 0.77year. Sample 4 was weathered similarly for 2.07 years. The recovery ofcarbon efficiency for trapping methyliodide-131 was outstanding in allcases as shown by the low penetration for the irradiated samples.

It has also been determined that exposures made to the gamma radiationfrom Co-60 at lower radiation levels, also increased the efficiency forthe trapping of methyliodide-131 (see Table 3).

                  TABLE 3                                                         ______________________________________                                        Penetration Before and After Co--60 Irradiation                               To Accumulated Levels of 10.sup.7 and 10.sup.8 RADS                           % PENETRATION OF METHYLIODIDE-131*                                            Sample  Original     10.sup.7 RADS                                                                           10.sup.8 RADS                                  ______________________________________                                        5       31.0         16.4      6.31                                           1       30.1         11.7      7.25                                           ______________________________________                                         *Standard Test ASTM/ANSI D380379                                         

This entailed measurements of the adsorption properties of theirradiated carbon samples. The penetration of non-radioactivemethyliodide-127 (MeI-127) was determined for many of the above carbonsamples before and after the irradiation. The results for two of thesamples are given in Table 4.

                  TABLE 4                                                         ______________________________________                                        Penetration of Methyliodide-127                                               and Methyliodide-131 for Service Carbons                                                       % PENETRATION                                                Sample             MeI-127  MeI-131                                           ______________________________________                                        1, Original Used Carbon                                                                           92      30.1                                              1, Irradiated (10.sup.9 Rads)                                                                     87      0.03                                              2, Original Used Carbon                                                                          138      10.5                                              2, Irradiated (10.sup.7 Rads)                                                                    103      1.9                                               ______________________________________                                    

The samples were coconut-shell charcoals in which the originalco-impregnation was potassium iodide and a tertiary amine. Sample 1 wasirradiated on the LINAC to an accumulation of 10⁷ rads.

The results (Table 4) demonstrate a great disparity between the behaviorof the radioactive and non-radioactive species. It was apparent that thecapacity for iodine isotope exchange has been vastly improved by theradiation exposure and that for adsorption was only slightly influencedor not at all.

High-energy radiation reacts with organic solids by processes known asradiolysis in which complex chemical decomposition reactions take place.By this invention, it has been determined that certain chemical reducingcompounds, added to the gas flow to a spent improved chemicallyimpregnated carbon, will likewise regenerate the iodine exchangecapacity of the exhausted chemically impregnated nuclear-grade carbon.In other words, the same regeneration can be accomplished either by theexposure to high-energy radiation or to the chemical reaction with avolatile reducing agent introduced into the air flow. The latter processis by far the one preferred and can readily be accomplished at ambienttemperature and in existing environments.

Detailed studies of the atmospheric weathering process reveals manycomplex changes within a nuclear grade carbon. It is instructive toobserve the penetration of radioactive methyliodide-131 after apertinent and specific compound is adsorbed on the activated carbonbefore and after radiation and before and after chemical reduction underambient conditions. One such compound is potassium iodidate (KIO₃). Aresult of the atmospheric degradation of a potassium iodide (KI)impregnated carbon is the oxidation of the iodide ion by atmosphericozone. The iodide ion is converted to iodate ion, wherein the iodine issurrounded by three oxygen groups to which it is chemically combined.The iodine isotope exchange between Na¹³¹ I and Na¹²⁷ IO₃ in neutralsolution is almost not measurable after a contact of 1,600 hours(estimated exchange half time greater than 5×10⁵ hour). One can inferthat once the iodide ion on activated carbon is oxidized, to iodate ion,the product would be quite resistant to iodine isotope exchange.

In order to identify this specific aspect, a coconut-shell charcoal(8×16 mesh) was impregnated with potassium iodate (analytical grade) inaqueous solutions and the concentration adjusted to realize 2 wt. % KIO₃on the weight of the activated carbon. The trapping of radioactivemethyliodide-131 was determined before and after exposure to the LINACradiation 10⁸ rads and before and after treatment with hydrazine, avolatile chemical-reducing agent. The adsorption of non-radioactivemethyliodide-127 was also determined under similar conditions using anelectron capture detector of high sensitivity.

                  TABLE 5                                                         ______________________________________                                        Penetration of Methyliodide (MeI) Through a                                   New Coconut Carbon Impregnated (2 wt. %) With KIO.sub.3                       Exposure        Test Gas % Penetration                                        ______________________________________                                        Original        MeI-131  6.0                                                  LINAC at 10.sup.8 Rads                                                                        MeI-131   0.13                                                Chemical Treatment                                                                            MeI-131  0.3                                                  ______________________________________                                    

The results (Table 5) show that both chemical treatment and radiationare successful in restoring the high efficiency of the carbon fortrapping radioactive methyliodide-131.

Other carbons removed from service were treated with diluted hydrazinevapors in air with quite satisfactory results. Some examples are asfollows:

    ______________________________________                                        Percent Penetration CH.sub.3 I-131                                                   Original                                                                             Treated                                                         ______________________________________                                               31     2.8                                                                    10     1.8                                                                    27.1   3.1                                                                    34.0   3.9                                                             ______________________________________                                    

Ozone is, therefore, one of the atmospheric contaminants that chemicallydegrade nuclear-grade carbons. The stoichiometry in a weak alkalisolution is:

    3 O.sub.3 +I.sup.- →3 O.sub.2 +IO.sub.3-.

If the same reaction takes place on a KI impregnated carbon, one wouldexpect to find KlO₃ in the weathered carbon.

The chemical reduction of KIO₃ in solution by hydrazine is as follows:

    N.sub.2 H.sub.4 +3N.sub.2 H.sub.4 →3N.sub.2 +2KI+6H.sub.2 O.

If the same reaction takes place on the weathered carbon, the formationof KI can readily be detected as shown above by iodine isotope exchange.

It has been determined that hydrazine and its derivatives as reducingagents are useful in the chemical reduction of specific groups absorbedon charcoal. Among the hydrazine derivatives used are:

Dihydrazine Sulfate--(N₂ H₄) 2H₂ SO₄

Hydrazine Hydrochloride--(N₂ H₄) HCl

Hydrazine Hydrobromide--(N₂ H₄) HBr

Monomethyl Hydrazine--(CH₃ NH) NH₂

Dimethyl Hydrazine--(CH₃)₂ N NH₂

Although there are a large number of reactions of hydrazine with manyorganic functional groups in the preparation of mono-, di-, tri-, andtetra-alkyl hydrazines, only those which are sufficiently volatile to beairborne either as a vapor or as an aerosol and thus be transferred to acarbon bed are useful in this invention.

It has been determined that new commercial carbons containing a KIimpregnation were not influenced by either radiation or chemicalreducing agents. A new activated carbon having both KI and TEDAimpregnations has at least three independent mechanisms for trappingradioactive iodine:

1. adsorption of the compound to the carbon network of the carbon,

2. iodine isotope exchange, and

3. chemical combination of the iodine with the tertiary amine.

For new carbons all three mechanisms are at peak performance, and it isnot possible to distinguish readily among them. However, as the carbonis weathered, each mechanism is subject to its characteristicdegradation process or processes. The present results show that anyprocess that maintains efficient isotope exchange is greatly to bedesired, since the bottom line in the nuclear application is a retentionof radioactive iodine by the carbon bed.

In this invention, the chemically impregnated carbons for filters havebeen improved in which tertiary amines form crystalline additioncompounds with alkyl iodides and their chemical properties aresignificantly different than the amine itself. The compounds in Table 6have been made and are presented as examples. The chemical analyses forcarbon, hydrogen and nitrogen (iodine by difference from 100%) are givenand compared with calculated values.

                  TABLE 6                                                         ______________________________________                                        Crystalline Addition Compounds of                                             Tertiary Amines and Methyl Iodide                                                             Calculated                                                                    Percent Found:  Percent                                       ______________________________________                                        Triethylenediamine.2CH.sub. 3 I                                                                 C      24.24  24.29,                                                                              24.10                                                     H      4.55   4.73, 4.68                                                      N      7.07   7.02, 7.05                                    (difference)      I      64.14  64.07                                         Hexamethylenetetraamine.CH.sub. 3 I                                                             C      29.79  29.73                                                           H      5.32   5.45                                                            N      19.86  19.80                                         (difference)      I      45.04  45.0                                          Quinuclidine.CH.sub. 3 I                                                                        C      37.94  37.82,                                                                              37.79                                                     H      6.32   6.82, 6.90                                                      N      5.53   5.45, 5.47                                    (difference)      I      50.2   49.88                                         ______________________________________                                    

It has been found that the above crystalline salts are soluble in waterand can be impregnated on charcoals to form very efficient materials foriodine isotope exhange. The determination of the penetration ofradioactive methyliodide-131 was made by the ASTM D-3803 test procedure.Some typical results are given in the second column of Table 7 in whichthe base charcoal was an activated coconut shell charcoal.

                  TABLE 7                                                         ______________________________________                                        Penetration of Methyl Iodide-131                                              Addition Compound   Penetration Percent                                       ______________________________________                                        Triethylenediamine.2CH.sub. 3 I                                                                   0.38                                                      Quinuclidine.CH.sub. 3 I                                                                          0.38                                                      Triethylenediamine.2CH.sub. 3 I                                                                   0.43                                                      Triethylenediamine.2CH.sub. 3 I                                                                   0.03                                                      Triethylenediamine.2CH.sub. 3 I                                                                   0.01                                                      Quinuclidine.CH.sub. 3 I                                                                          0.08                                                      Quinuclidine.CH.sub. 3 I                                                                          0.04                                                      Hexamethylenetetraamine.CH.sub. 3 I                                                               0.23                                                      Hexamethylenetetraamine.CH.sub. 3 I                                                               0.47                                                      ______________________________________                                    

The above results are for independent preparations using three differentsources for the coconut shell charcoals. Equivalent impregnations can bemade using coal-base charcoals.

The above impregnations of the tetraalkylammonium iodides can beefficiently regenerated by hydrazine after the charcoal was degraded bythe contaminants of the air.

In carrying out this invention for regenerating an activated chemicallyimpregnated carbon filter in situ, a controlled quantity of the chemicalreducing agent is introduced into the air flow at a suitable place inthe duct work from the nuclear containment space and which is thendirected through the carbon filter for a period of time from about 20minutes to about six hours. Ambient temperatures up to 50° C. can beused. An aerosol of about 1 to 20 microns in size may be used. Lowconcentrations of hydrazine can be used for longer periods because ofthe accumulation effect in the chemically impregnated activated carbon.It is preferred to carry out the treatment at intermittent intervals (upto six months) so that the chemically impregnated activated carbon doesnot degrade excessively between treatments and thus require lessreducing agent.

The hydrazine concentrations can be kept low (100 ppm) and it has beendetermined that the hydrazine, not reacting with the contaminant, willdecompose during the passage through the carbon bed so the the effluentcontains no hydrazine. The reaction products consist of nitrogen andwater vapor.

The chemical reducing agent reduces the oxidized impregnant, forexample, to KI originally on the charcoal, so that it is restored to theoriginal noncontaminated state. It has been determined that suitablechemical reducing agents are those which are sufficiently volatile to beairborne either as a vapor or an aerosol so that they can be conductedby the air flow via the duct work between the reactor containment spaceand the carbon filter. Suitable chemical reducing agents are hydrazineand its derivatives and hydroxylamine and its derivatives.

I claim:
 1. An activated charcoal filter impregnated with a compoundselected from Quinuclidine.CH₃ I; Triethylenediamine. 2CH₃ I; andHexamethylenetetramine.CH₃ I for efficiently removing radioactive iodinefrom an air flow directed through said impregnated filter.
 2. Anactivated charcoal filter as set forth in claim 1 wherein said compoundis Triethylenediamine.2CH₃ I.